Automatic gain control (AGC) is an adaptive system found in many electronic devices. Typically, the average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels. AGC has been long-applied in receiver technologies.
For example, without AGC the sound emitted from an amplitude modulated (AM) receiver would vary to an extreme extent from a weak to a strong signal. The AGC effectively reduces the volume if the signal is strong and raises it when it is weaker. AGC technologies can also be applied, for example, to frequency modulated (FM) receivers and phase modulated (PM) receivers.
Digital receivers are designed to amplify and detect digitally encoded signals. Such receivers may also be paired with digital transmitters to form digital transceivers. In some instances, the digital receivers can be multi-mode, such as digital AM receivers and digital FM receivers. Various AGC systems have been designed for such digital receivers.
Since AGCs operate by feeding back some of the output of a stage to control the gain, they are often referred to as having “AGC loops.” AGCs are of three general types: 1) all-digital; 2) analog; and 3) mixed-mode including both digital and analog components.
U.S. Patent Publication 20070229340 of Krishnamoorthi et al. describes an all-digital gain control system. However, a fully digital implementation such as that described by Krishnamoorthi et al. requires a very wide dynamic range front end, including the ADC. Such a system would likely have substantial power requirements due to the requirement of the very wide dynamic range.
U.S. Pat. No. 5,483,691 of Heck et al. describes an analog AGC loop with dual control ports. However, analog loops are slow to settle since the gain control feedback delay must be much greater than the signal path delay to ensure stability. An analog loop can be substantially slower than a mixed-mode loop, depending on the initial conditions of the stability capacitor and the strength of the input signal.
U.S. Pat. No. 5,742,899 of Blackburn et al. describes another analog loop with optimized initial conditions on the feedback capacitor to improve attack time. However, a large feedback capacitor is required to stabilize the AGC loop and thus the charging and discharging of the feedback capacitor would slow the AGC settling time.
U.S. Pat. No. 6,417,730 of Seagallis et al. describes a mixed-mode control loop where the controller manipulates the gain of a plurality of variable-gain stages to get within the linear range of a sensor. This patent describes a system which “hunts” for a desired operating level for the AGC by incrementally adjusting gain. This results in a variable settling time for the AGC loop. Furthermore, this system has an attenuator which is not linear in dB, making it difficult to account for the attenuator setting when calculating RSSI (Received Signal Strength Indication, a measure of the power present in a received radio signal).
U.S. Pat. Nos. 6,654,594 and 6,885,582 of Hughes et al. describe mixed-mode AGC loops with various features. For example, attenuator step-size during acquisition can be varied to help improve attack time and an out-of-band detector can be added to account for overload conditions. However in both of these embodiments, the controller again hunts for the optimum gain control setting as it steps through various attenuation settings and then measures the results, slowing the settling rate of the AGC loop.
U.S. Pat. No. 5,684,431 of Gilbert et al. describes a Bipolar Junction Transistor (“BJT”) implementation with “active feedback” and U.S. Pat. No. 5,200,625 of Feldt describes another BJT implementation. U.S. Pat. No. 7,403,071 by Hollenbeck et al. describes an RF CMOS variable gain amplifier and U.S. Pat. No. 7,391,260 of Kim et al. describes another all-CMOS circuit. The control characteristic for these four references is linear-in-dB with respect to a continuous control voltage. However, to implement a mixed-mode control system with these circuits would require a voltage DAC, which consumes power and may degrade settling time. Also, CMOS solutions typically have only approximately 25 dB of linear control dynamic range.
It will be appreciated that while there are a variety of AGC circuits in the prior art, that all suffer from one or more limitations. What is needed is a low cost, low power, fast-attack AGC solution that avoids incremental automatic gain control whenever possible.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.